1. Field of the Invention
This invention relates to a film wiring board.
2. Description of the Related Art
A film wiring board on which electronic parts such as IC chips are directly mounted has such a structure, for example, as shown in FIGS. 5(A) and 5(B). Regarding this film wiring board 1, wiring patterns 4 of metal such as copper are formed via a bonding agent 3 on an upper surface of a film base 2 made of resin, as explained below. Connection pads 4a in almost circular shape and lead wires 4b which are lead out from the connection pads 4a are formed at predetermined portions of the wiring patterns 4. Protective film 5 of resin such as epoxy is provided entirely on exposed upper surfaces of the bonding agent 3 and the wiring pattern 4 excepts for central portions of upper surfaces of the connection pads 4a. Thus, almost circular openings 6 are formed in the protective film 5 at the central portions of the upper surfaces of the connection pads 4a. As will be explained below, when connection electrodes of an IC chip are connected via soldering bumps to the connection pads 4a exposed through the openings 6, the IC chip is mounted on the film wiring board 1.
Next, manufacture of the conventional film wiring board 1 will be described with reference to FIGS. 6(A)-6(C). First, as shown in FIG. 6(A), a metal foil 8 for forming the wiring patterns 4 (see FIG. 5(A)) is bonded on the upper surface of the film base 2 via the bonding agent 3. Then, an upper surface of the metal foil 8 is coated with a photoresist 9 having a predetermined pattern corresponding to the wiring patterns 4, in a well-known method of photolithography. When the metal foil 8, with the photoresist 9 being used as an etching mask, is etched by means of wet etching using copper chloride solution or ferric chloride solution, the wiring patterns 4 having the connection pads 4a and the lead wires 4b are formed as shown in FIG. 6(B) and FIG. 5(A). Thereafter, the photoresist 9 is stripped. As shown in FIG. 6(C), the protective film 5 is formed by coating an epoxy resin or the like on the exposed entire upper surface of the bonding agent 3 and the wiring patterns 4 in a method such as spin coat or roll coat and drying the coated resin, and the photoresist 10 of a predetermined pattern having openings 10a corresponding to the openings 6 (see FIG. 5(B)) is coated on an upper surface of the protective film 5. When the protective film 5 is etched by a well-known method, with the photoresist 10 being used as an etching mask, the almost circular openings 6 are formed in the protective film 5 at the central portions of the upper surface of the connection pads 4a, as shown in FIGS. 5(A) and 5(B). After that, the photoresist 10 is stripped. Thus, the film wiring board 1 is produced.
Mounting of an IC chip on the film wiring board 1 by a flip-chip method will be described with reference to FIGS. 7(A)-7(C). First, as shown in FIG. 7(A), at a portion of the upper surface of the film wiring board 1 where an IC chip is to be mounted, a flax 11 is painted by a letterpress printing. This flax 11 is used to fix temporarily the IC chip. Then, as shown in FIG. 7(B), soldering bumps 13 provided in advance at connection electrodes 12 of the IC chip 7 are positioned on the connection pads 4a at the openings 6 (see FIG. 5(A)), and are temporarily fixed by the flax 11. Then, the connection electrodes 12 are pressed on the connection pads 4a while the electrodes 12 are heated by means of, for example, a thermal press head (not shown), the bumps 13 are melted, bonded to the connection pads 4a, and solidified, and then, the connection electrodes 12 of the IC chip 7 are connected to the connection pads 4a via the soldering bumps 13. Thereafter, the flax 11 is removed by a well-known method such as cleaning with of a solvent, and the resultant structure is sealed with a sealing compound 14 of resin such as epoxy, as show in FIG. 7(C). Thus, the IC chip 7 is mounted on the film wiring board 1.
In the conventional film wiring board 1 as described above, since the openings 6 are formed by a photolithography method in the protective film 5 coated on the entire surface of the film base 2 including the wiring patterns 4, the conventional board 1 requires more steps such as the protective film coating, photoresist coating, light exposing, development, etching, photoresist stripping, etc., as compared with a case where, for example, the protective film 5 having the openings 6 is formed in one step by a screen printing.
Explained here is a reason why the openings 6 are formed by the photolithography method in the protective film 5 coated on the entire surface of the film base 2 including the wiring patterns 4. Since the metal foil 8 bonded via the bonding agent 3 on the upper surface of the film base 2 is wet-etched with the photoresist 9 being used as an etching mask so that the wiring patterns 4 having the connection pads 4a and the lead wires 4b are formed, it takes much time to etch a bottom portion of the metal foil 8, contacting with the bonding agent 3, in the wet-etching process. Therefore, if the width of the photoresist 9 is decreased, all the upper portions of the metal foil 8 will be etched while the bottom of the metal foil 8 will be etched, and it will be impossible to make the width of the photoresist 9 i.e., the width of the lead wire 4b of the wiring patterns 4 smaller than 50 .mu.m. The width of the lead wire 4b in the conventional film wiring board 1 is limited in the range of at most 50 .mu.m-60 .mu.m. In this case, if a diameter of each of the connection pads 4a is determined at 100 .mu.m-140 .mu.m and the connection electrodes 12 of the IC chip 7 are connected via the soldering bumps 13 to these connection pads 4a, there is a possibility that the soldering bump 13 once melted at the time of thermal press flows from the corresponding connection pad 4a to the corresponding lead wire 4b, further from the lead wire 4b to its adjacent lead wire 4b, and thus, short-circuit may occur between adjacent lead wires 4b, because the width of the lead wire 4b is comparatively large, i.e. at 50 .mu.m-60 .mu.m.
In order to prevent the flow of the solder from the connection pad 4a to the lead wire 4b, the diameter of the opening 6 of the protective film 5 is made to be slightly smaller than that of the connection pad 4a, for example, at 60 .mu.m-100 .mu.m, so that the inner wall of the opening 6 prevents the molten solder from flowing out. However, if the diameter of the opening 6 of the protective film 5 is small or fine, i.e. at almost 60 .mu.m-100 .mu.m, the protective film 5 having the openings 6 cannot be formed by the screen printing and it must be formed by the photolithography method. As a reason for this, the diameter dimension of the smallest opening in the screen is at almost 200 .mu.m and its positional accuracy is smaller than .+-.300 .mu.m while in the case of the photolithography method, the diameter dimension of the smallest opening is at almost 10 .mu.m and its positional accuracy is smaller than .+-. 20 .mu.m.
As understood from the above, the conventional film wiring board 1 in which the openings 6 are formed by the photolithography method in the protective film 5 coated on the entire surface of the film base 2 including the wiring patterns 4 requires more production steps, decreases its productivity, and needs much manufacturing cost, in comparison with a case in which the protective film having the openings 6 is formed in one step by the screen printing.